A cache memory is a memory that is packaged as an integral component of a processing unit in a computer system. The cache is generally much smaller than main memory. Its purpose is to serve as a buffer between the processors and the memory, emulating the main memory, but with a much faster access time.
For multiprocessor systems, the cache management strategy also includes algorithms that provide a coherent view of storage to all of the processors in the system. Coherency implies that store operations to a memory location performed by a given processor (e.g., a server) will be made consistent with fetch operations done to the same memory location by another processor (e.g., a client). Coherency provides a means for determining when cached data in a given processor becomes obsolete as a result of store operations performed by another processor.
In the Andrew file system, for example, the server maintains a record of which data is cached in each of the clients. Typically, coherency is maintained by providing a valid bit for each datum in each respective client's cache. A "cross invalidate" (XI) is the act of invalidating, or marking non-resident, a line in the cache of a remote processor. When a server needs to change the value of a datum, the server broadcasts XI messages to all of the other processors that may have a copy of the same datum in cache. If a copy of the datum is present in one of these other caches, that copy is marked invalid (e.g., the valid bit is reset) in response to the XI. Only after all copies are marked invalid does the first processor change the target data value. A request for the datum then results in a cache miss.
In the Network File System, the server does not have to keep track of which clients have copies of each respective datum. Whenever a client needs to access data in its respective cache, it queries the server to verify that its copy of the data is current.
The paradigms described above work well when all of the processors in the system are active at the same time, and when the communications paths between the processors are static. Typically, this has been the case when all of the processors are collocated and are operated continuously.
The introduction of wireless cellular communications and palmtop computers into the marketplace introduces new capabilities and also poses new technical challenges. The need to share data in this distributed and mobile environment presents one of these challenges. It is desirable to share data among fixed location mobile unit support stations and mobile palmtop units.
The conditions in a mobile wireless computing environment differ from those encountered in the static, collocated multiprocessor systems described above. In the mobile environment, a large number of users equipped with low powered palmtop machines may query databases over wireless communications channels. Palmtop units are often powered down (taken off-line) for prolonged periods of time to conserve battery energy. Thus, if palmtops are equipped with caches, the palmtops may not always be available to receive cross invalidate messages from the mobile unit support stations, if such messages are sent.
Furthermore, the palmtop users do not maintain fixed or universally known positions in the wireless network. A given palmtop unit may be in communication with different mobile unit support stations at different times. The mobile unit support stations cannot predict which palmtop units will be within their respective radio coverage areas at any given time.
Although the mobile unit support stations may be located in proximity to one another, for example, within a single building or campus, it is contemplated that palmtop machines will also communicate over conventional cellular communications networks as well. In the latter case, the communications bandwidth may be limited (e.g., 10 to 20 kilobits per second). The bandwidth places a limit on the number of queries to which the server can respond in a given period of time. Because of the limited bandwidth in the cellular environment, it is impractical for each palmtop to query the mobile unit support station for a complete database refresh each time the palmtop user wishes to access data after returning to the on-line state. The mobility of the palmtops, their frequent unavailability to receive XI messages, and communications bandwidth limitations make caching of data within the palmtops by the conventional paradigms difficult.
The above identified factors tend to make communications with palmtop machines more complex. At the same time, some of the constraints that have driven the design of many prior art systems may not apply in some of the applications for palmtop computers. For example, palmtops may be used by consumers to access data that are updated with a frequency that is much smaller than the frequency of queries sent from the clients to the server. Additionally, some palmtop applications may tolerate a small, known probability that the data in the palmtop are considered to be current when in fact the data have been updated (For example, when updates are infrequent and are minor). For such applications, the prior art cache management strategies for distributed systems may be inefficient.
An improved method for maintaining a coherent view of the data in the cache of each mobile unit is desired. Desirably, the improved method would not require the mobile units to stay on-line at all times, and would not require a full cache refresh each time a mobile unit is turned on.